Some X-ray image diagnostic apparatuses have an Automatic Brightness control System (hereinafter, abbreviated as “ABS”) that controls a tube voltage automatically so that the brightness of a fluoroscopic image is always constant even if an object thickness is changed. As an example of the ABS, there is a system that sets a region of interest (hereinafter, abbreviated as “ROI”) for an image region output from an X-ray detector and controls a tube voltage automatically such that a value of a feedback signal approaches a reference value set in advance using an average brightness value in the ROI as the feedback signal. As an example of a control method, for example, when the feedback signal is lower than the reference value, the tube voltage acts in an increasing direction. As the tube voltage increases, the X-ray output increases. Accordingly, the next fluoroscopic image becomes an image having a higher brightness than an image before the increase.
However, when the position of the ROI is fixed, if the living tissue of an object that an operator desires to see deviates from the ROI for some reasons, such as the position movement of the object, an appropriate feedback signal is not calculated. Accordingly, it is not possible to obtain the preferred image quality.
In order to eliminate the undesirable effect described above, PTL 1 discloses a fluoroscopic apparatus that sets an ROI freely with a pointing device, such as a mouse, during X-ray fluoroscopic imaging and feeds the result back to the ABS using image data in the ROI.